This invention relates to the control of the piston of a rotary piston engine with a single-arc trochoid as housing runway.
In rotary piston engines such as the Wankel engines, the guidance of the piston kinematics normally takes place via a large internal gear. Such gear is placed in the piston at a housing side wall and mates with a smaller toothed wheel. At the same time, the eccentric shaft for the power take-off of the engine is guided through the smaller toothed wheel. The piston is arranged on a central journal bearing on the eccentric shaft in such a way that the piston can turn around the power shaft and, simultaneously, caused by the meshing of the gears, turns around itself. In the well-known Wankel engine the diameters of the toothed wheels, internal gear in the piston and external gear at the housing wall, have a ratio of 3 to 2, thereby forming a double-arc trochoid as the housing runway.
Rotary piston engines having a housing runway of the shape of a single-arc trochoid are especially suited for large changes in volume. Here the ratio of the diameter of the internal gear in the piston to the diameter of the external gear at the housing wall is 2 to 1. The piston of the engine has a biangular shape. A disadvantage, however, is that with an unsuited arrangement of the openings for the fluid change, short circuit flows may take place between inlet and outlet. These short-circuit flows can be avoided by having the fluid change take place via side openings in the housing side wall. However, the biangular piston has only a small area and it is difficult to arrange the side openings in such a way that they can be simultaneously opened and covered by the movement of the piston.
This difficulty can also be found in similar engines which are not rotary piston engines in the true sense of the word. An example for such a type of engine is the rotary piston engine of the Australian company Katrix Pty Ltd. An unfavourable feature of such engine is the fact that piston and power shaft are connected by a sliding guidance. In such a case it is, however, possible to select any housing runway as long as the piston rotation allows the points of the piston always to be conducted along the runway contour. When the points are always conducted along the runway contour, however, the resulting fluid power goes via the sliding guide on to the power conducting shaft. The consequences of this arrangement are high friction in the sliding pairs combined with high wear of the components. On the other hand, the resulting power of a rotary piston engine always acts on the eccentric so that in this case the power shaft leading through the engine can be dispensed with.
Another known guidance of the piston kinematics in rotary piston engines with a housing runway of the shape of a single-arc trochoid is arranged as is shown in FIG. 1. A special feature of this rotary piston engine is that the transmission of both toothed wheels is at a ratio of 2 to 1. According to the mathematic formation law, an imaginary vertical axis 6 going through a piston 1 always goes through a point 3 fixed to the housing 2; and a horizontal axis 7 going through a piston always goes through a point 4 fixed to the housing. Points 3 and 4 are at the same time points in a Cartesian coordinate system with the axes 8 and 9. For a power shaft with the centre 5 it is of no importance whether the rotation of the piston around itself is caused by the interaction of two toothed wheels 10 and 11 or by the sliding movement of the piston through the points 3 and 4.
In each case, resulting fluid power at the piston always goes through the eccentric centre point and has a lever arm to the centre of rotation 5 of the power shaft. The eccentricity of the rotary piston engine is the distance of the points 3, 4 to the centre 5. The tips of the piston stay free of the guiding forces. This kinematic principle has already been described in German patent DD 95574 A.
FIG. 2 shows that other rotating points can be chosen at the housing side wall for the purpose of a rotary sliding guidance. In FIG. 2, the axes 12 and 13 are running through the rotary sliding points 14 and 15. The axes 12 and 13 are turned towards the symmetry axes by an angle in FIG. 1. This angle can be chosen ad libitum according to the position chosen at the housing side wall for the rotary sliding points.
Although the guidance of the kinematics of the piston of a rotary piston engine with a single-arc housing contour with toothed wheels in the piston side presents an elegant and safe solution, a large area is occupied by a through power shaft. Also, a large area is occupied by a non-through power shaft. This is due to a positioning of a large internal gear next to the eccentric, which makes this space unavailable for the change of the fluid at the piston side area.